1. Field of the Invention
The present invention relates to multilayer mirrors, particularly to beryllium based multilayers for extreme ultraviolet (EUV) lithography optics, and more particularly to a high reflectance, low intrinsic roughness and low stress multilayer composed of MoRu/Be, which may include a diffusion barrier on the interfaces and/or a capping layer.
2. Background
Beryllium (Be) based multilayers reflect at wavelengths above 11.1 nm. At these wavelengths, the spectral match with a laser-plasma EUV source improves the optical throughput by a factor of five, as compared to the currently used Mo/Si wavelength (13.4 nm). Higher throughput is crucial for commercial success of EUV lithography because it reduces the costs.
Molybdenum/beryllium (Mo/Be) multilayers have been studied as potential candidates for EUV applications, as discussed in Skulina et al., "Molybdenum/beryllium multilayer mirrors for normal incidence in the extreme ultraviolet," Appl. Opt. 34:327-3730 (1995); and Stearns et al., "Beryllium-based multilayer structures," Mat. Res. Soc. Symp. Proc., Vol. 382, 1995 Materials Research Society. A 70% reflectance using Mo/Be coatings with 70 periods (bilayer pairs) at 11.3 nm and 0.28 nm peak width has been demonstrated. Even though Mo/Be multilayers have higher reflectance than Mo/Si multilayers, they have a disadvantage: the effect of a rough (&gt;0.2 nm RMS) substrate on their reflectance. A rougher substrate induces a substantial increase in the roughness of the Mo/Be multilayer. See D. G. Stearns, "Intrinsic defects in multilayer coatings: A statistical analysis," (1998). The reason for this is the crystallinity of both Mo and Be layers. Because both layers are crystalline, there is no mechanism to heal imperfections on the substrate or defects occurring during the growth of the multilayer. This leads to a much higher intrinsic roughness in Mo/Be multilayers as compared to Mo/Si multilayers.
Also, a typical Mo/Be multilayer consists of 70 bilayers (or periods). A peak width is dependent on the number of bilayers and on the optical properties of the two materials in the multilayer. A smaller number of bilayers is preferable because the peak width is then larger. For example, a typical Mo/Si multilayer coating consists of only 40 bilayers and therefore the peak width is about 0.5 nm. The peak width of a 70 bilayer Mo/Be coating is 0.28 nm. Thus, there is a need for a high reflectance, low intrinsic roughness and low stress multilayer system for EUV lithography optics which has a low number of bilayers and a high peak width.
The present invention addresses this problem by providing a multilayer system with a reflectivity of above 70% and a peak width 0.35 to 0.45 nm with 50 bilayers. The multilayer system of this invention is composed of amorphous layers of an alloy composed of molybdenum and ruthenium, and crystalline beryllium layers, thus resulting in a multilayer of molybdenum ruthenium/beryllium (MoRu/Be), with an ideal theoretical peak width of 0.45 nm and reflectivity of over 75%. The period (bilayer) thickness is typically 5.85 nm and the multilayer reflects at 11.4 nm. This multilayer is much thinner than the 70 bilayer Mo/Be and is of comparable thickness to 40 bilayer Mo/Si multilayer. Therefore, the image quality due to thickness uniformity will be comparable to Mo/Si multilayer optics. The MoRu/Be multilayer system has a very low intrinsic stress (21 MPa tensile stress for a 50-bilayer system). To increase the peak width and improve the throughput, a diffusion barrier can be added to make the interfaces sharper. Also, a capping layer can be utilized to improve the long term reflectance stability of the multilayer.